This invention relates to a method and apparatus for the continuous regulation of rotary hydrodynamic pumps which can be driven at variable speeds.
Several forms of variable speed pumps are known in the art. In such pumps, the speed is controlled by means of a speed regulator in accordance with a command variable. The purpose of the control may be to maintain the pump pressure constant, to maintain the level in an elevated tank constant, or to maintain a constant flow rate, for example. In some cases a parameter in a process, such as the temperature of a thermal process or the concentration of some substance in a chemical or biological process, may be used as the command variable.
In order to achieve or to increase the required stability of a control system, it is generally known to provide a strong feed-back signal related to the quantity to be controlled, i.e., the pump speed in the present instance. The pump speed is usually measured electrically, e.g., by a tachometer generator, or in an indirect electrical manner, e.g., by means of photocells and a rotatable disc having black and white fields. When the primary purpose of the pump is the transfer of liquid, i.e., when the pressure created is incidental, a feed-back signal related to the pump flow rate may be the means of stabilizing the control system. In such a case, the flow rate has to be measured hydraulically or electrically by means of the pressure drop in a venturi pipe or in an orifice plate, with the aid of a device commonly called a dp-cell. The flow rate may also be measured by means of other types of flow meters based, for example, on a magnetic, capacitative or ultrasonic effect.
Certain special cases of control systems for rotary, hydrodynamic, variable speed pumps are known. For example, there are known controlled pump units which automatically adapt their performance in accordance with the instantaneous conditions in systems where the control requirement depends on the opening and closing of valves at arbitrary positions in the pipe system. A specific example of such a pipe system is a system for the distribution of drinking water, where pump operation is controlled in accordance with the opening or closing of one or several tap valves. The automatic control effect in such a system is obtained by means of a control system having a piston which senses the pressure drop across a minor restriction (i.e., a quantity proportional to the square of the flow rate) and which is biased by a force from a weight or a spring. This force corresponds to the static head of the whole pump system, i.e., the sum of the differences in the geodetic head and the pressure head between the locations of liquid transfer.
Since the measurement of the liquid flow with sufficient accuracy in a flowmeter is very expensive or may, in practice, be impossible to perform, e.g., due to pollution of the liquid, electronic systems using indirect measurement have been designed. In such systems, the pump speed is measured electrically, whereafter the square of the signal is obtained electronically and thereafter reduced by a term corresponding to the static head of the pump system. The signal obtained in this way is then at least approximately proportional to the square of the flow rate and is used for a feed-back signal in the control system.